In the normal breathing process, inhaled air becomes warmed and humidified as it passes through the nasal, tracheal and bronchial passages. This basic body function protects the delicate membranes in the lungs, but may not be sufficiently effective in heavy or rapid breathing of very cold dry air. During exhalation, some heat and moisture is returned to the walls of the breathing passages, but most of the heat energy and moisture is lost in the exhaled gases.
At rest and at a comfortable room temperature, the energy loss is on the order of 1 Kcal per hour, and is easily compensated by normal body functions. However, as an example, at a temperature of -30.degree. C and at an altitude of 5,500 meters, with a low humidity and at a moderate working rate of 60 breaths a minute, averaging about 2 liters per breath, the loss would be about 230 Kcal and 250 grams of water per hour. This is a significant portion of the body's energy output and the mere use of warm clothing may not be sufficient to retain a desirable amount of the energy. Also, since thirst response is suppressed by extreme cold, dessication could become a problem.
Various techniques have been developed for heating and humidifying breathing gas, but are usually complex and heavy. Thermal heaters require power sources and are not particularly efficient in their use of energy. In a dry atmospheric environment, a humidifier must contain stored water in some form and is thus heavy and bulky. For convenience and reliability, such apparatus should be simple, compact and require a minimum of storable energy producing medium.